It has long been recognized that cardiac defibrillation can be accomplished through application of an electrical shock to the cardiac muscle. See, for example, Swartz et al., “Influence of T-Wave Shock Energy on Ventricular Fibrillation Vulnerability in Humans,” Journal of American College of Cardiology, 1995 Conference Abstracts, February 1995; see also, Karolyi et al., “Timing of the T-Wave Shock for Inducing Ventricular Fibrillation in Patients With Implantable Cardioverter Defibrillators,” PACE NASPE Abstracts, Vol. 18, April 1995 (Part II), p. 802.
Numerous types of defibrillating devices, both external and implantable, are available for the purpose of cardiac defibrillation through electrical stimulation. One example of an implantable cardiac defibrillator (ICD) is the Medtronic® Gem® III DR, Model 7275 ICD, commercially available from the Assignee of the present invention.
During the implantation process and subsequent periodic followup sessions, the physician tests the capabilities of the ICD to ensure its ability to provide therapy. One method of testing a defibrillator's capability to reliably defibrillate the heart involves induction of an episode of fibrillation in the patient's heart, and then allowing the ICD to detect and terminate the induced fibrillation. The ICD itself has the capability of inducing fibrillation both during the implantation procedure or, later, during regular follow-ups. A command to start an induction, as well as other associated parameters required for a successful induction and therapy, are downloaded to the ICD via a programmer such as the Medtronic® Model 9790 programmer.
It is known in the art that fibrillation can be induced in either chamber of the heart (atrial or ventricular) by delivering a stimulus during that chamber's repolarization phase. In other words, delivery of electrical stimulus is preferable within a so-called “vulnerability window” following the chamber's depolarization period when the heart has begun to repolarize but has not completely repolarized. This process is disclosed, for example, in U.S. Pat. No. 5,129,392 (the '392 patent) issued to Bardy et al., which is incorporated herein by reference in its entirety.
According to the '392 patent, the pulse intended to induce fibrillation is delivered in a timed relationship to an immediately preceding pacing pulse. An overdrive pacing and capture detection protocol or equivalent is carried out to determine the patient's Q-T interval, enabling a subsequent fibrillation-inducing shock to be delivered at a time known to fall near the end of this interval but prior to the conclusion of the repolarization phase. The method and apparatus, disclosed in the '392 patent, are believed to allow for extremely accurate placement of the fibrillation-inducing shock relative to the refractory period of the patient's heart.
The Medtronic® Gem® III ICD is an example of a commercially available device that is capable of delivering a stimulus during the repolarization phase to induce fibrillation. As is well known in the art, such devices have various programmable features. For example, the Gem III's T-Shock feature requires programming of the following major parameters that include: the rate at which overdrive pacing pulses are delivered to pace at a rate known to be above the patient's intrinsic cardiac rate, the amplitude of fibrillation inducing shocks, and the shock coupling interval relating to the interval from delivery of the last overdrive pacing pulse to the delivery of the fibrillation-inducing shock.
Some clinicians may regard programming several parameters to effect automatic fibrillation induction as inconvenient or undesirable, especially if these parameters require access from several different programmer screens. Although a set of nominal or default parameters can be specified for the device, such nominal parameters may not be appropriate for some patients, such as those on anti-arrhythmic drugs that slow conduction.
In the interest of implant efficacy and time constraints, it is desirable to have the ability to program all the various required induction and defibrillation parameters from a single task-oriented induction screen.